There are a wide variety of commercially available computer programs which facilitate transferring files between computers utilizing modems and telephone lines. Among the commercially available program which provide such functions are: "crosstalk" marketed by DCA Corp of Atlanta, Ga.; "QModem" marketed by Forgin Inc. of Cedar Falls Iowa; and, "Close-Up" marketed by Norton Lambert Corp of Santa Barbara, Calif.
The physical characteristics of normal telephone lines limit the transmission speed which can be used to transmit data over such lines. In order to shorten the time required to transmit data, various types of data compression and error correcting protocols are in widespread use.
Where a system transmitting a data file from a first computer to a second computer is merely updating a file which has been previously transmitted to the second computer, the transmission time can be shorted by merely transmitting information which covers the "differences" between the present file and the previously transmitted file. This technique of only transmitting the "delta" between two files is only applicable in situations where the sending system knows the state of the file at the receiving station.
The present invention provides a technique for transmitting files between computers where the computer receiving the information has a file related to the file being transmitted, but where the sending computer does not know the state of the file at the receiving computer.